Various tests have verified the utility of electro-dynamic tethers in space such as the Small Expendable Deployer System (SEDS 1 & 2), the Plasma Motor Generator (PMG) and the Tethered Satellite System flights (TSS-1 & TSS-1R). Electro-dynamic tethers, in a relatively vertical gravity-stabilized manner, interact with the magnetic fields of the Earth's or other celestial bodies magnetic fields to produce either electrical power or thrust. These tethers generally consist of a wire extending from a satellite or connected between two satellites, each containing plasma contactors, which are orbiting the Earth or other celestial body. An electromotive force (emf) is induced across the length of the tether.
The emf acts to create a potential difference across the tether by making one end of the tether positive with respect to the other end. In order to produce a current from this potential difference, the tether ends must make electrical contact with the Earth's plasma environment. Both plasma contactors and large conductive surfaces at the ends of the tether provide this contact, establishing a current loop through the tether, external plasma and the ionosphere around the Earth, sometime called a phantom loop. An example of the phantom loop is shown in FIG. 1. Two field lines 13a and 13b representing levels of the magnetic field of the Earth 12 are shown. As the tether 100 connects the field regions 13a and 13b of the Earth 12 in the orientation as shown, the electrons are moved towards the other end, near plasma contactor 300, of the tether, thus charging the ends of the tether, positive near a plasma contactor 200 and negative near plasma contactor 300. When these plasma contactors or conductive surfaces are placed on the ends of the tether, the electrons are free to travel into and out of the tether cable creating charged clouds. As shown in the figure, the collection of electrons from a positive end plasma contactor 200 and their emission from a negative end plasma contactor 300 creates a net positive cloud 14 at the positive end plasma contactor 200 and a negative cloud 15 at the negative end plasma contactor 300. The excess free charges migrate along the geomagnetic field lines intercepted by the tether ends until they reach the vicinity of a lower section of the ionosphere E where there are sufficient collisions with neutral particles to allow the charges to migrate across the field lines and complete the phantom circuit.
Once the current is established within the tether, the combination of the current running through the tether, the tether orbiting about the earth and the geomagnetic field of the Earth react together to create a force that acts on the tether. This force acts in a direction opposite to the movement of the tether across the magnetic field in orbit. An example of this type of electro-dynamic tether is shown in U.S. Pat. No. 6,116,544 (Forward et al.), incorporated herein by reference in its entirety. The tether consists of a wire attached to a spacecraft at one end and attached to an end mass at the other end. The tether is made of a braided aluminum or copper wire. The tether is used to slow the spacecraft down and reduce its orbit.
Another application of the tether is to generate power to either charge batteries on the spacecraft or power spacecraft subsystems. As outlined above, the orbit of the tether across the Earth's magnetic field induces a force on the electrons within the tether wire, which creates a charge separation and produces an electric potential due to Coulomb's law until the forces are balanced by current flow. When the current flow stops, there is a potential difference between one end of the tether and the other. After completing the circuit, the power provided may be used to power spacecraft substations or charge batteries. An example of such a system is shown in U.S. Pat. No. 4,923,151 (Roberts et al.), incorporated herein by reference in its entirety, with the use of a tether extended between two satellites. The tether comprises outer conductive material layers and an inner conductive material layer electrically connected to the tethered object, both separated by an insulator material.
Studies have shown that the use of a bare wire in space would substantially increase the collection of electrons as opposed to the use of conventional plasma contactors. However, a problem with bare wire plasma contactors is that in an oxygen rich environment of a low Earth orbit, the bare wire tethers of copper or aluminum would quickly oxidize and degrade, losing their electro-dynamic properties. Such greatly reduces the effective life of the tether and would require more frequent replacement. Secondly, the Earth's thermal albedo would raise the temperature of the bare wire tether to the point that its resistance would rise, thus decreasing its performance and reducing effective use of the tether.
Another problem with the prior art devices is that the wire constructions are not sufficiently flexible enough to allow a sufficient length of the tether to be wound up in a relative small space on the satellite or spacecraft orbiting a celestial body.